Quasi-phase matching (QPM) devices for optical parametric frequency conversion have been studied for applications to high-speed all-optical signal processing due to their ultrafast nonlinear optical response and wide conversion bandwidth [1, 2]. Among the several ferroelectric crystals which can be periodically poled, LiNbO₃ is the most promising candidate for QPM devices due to its outstanding nonlinear optical properties [3]. Periodically poled LiNbO₃ (PPLN) waveguide devices have been widely spotlighted for all-optical signal processing such as wavelength conversion [2] and switching [4, 5], because they satisfy numerous requirements of fiber optical communications such as independence of data rate and -format, low cross talk, and no intrinsic chirp. In particular, all-optical channel dropping is one of the key functionalities in future reconfigurable photonic networks. Optical parametric switching based on sum frequency generation (SFG) has been demonstrated in several different PPLN samples such as bulk PPLN, annealed proton exchanged (APE) PPLN waveguide and Ti-indiffused PPLN waveguide [6, 7]. This optical parametric switching can perform not only time-selective but also wavelength-selective channel dropping [7]. Up to now, wavelength- and time- selective single channel dropping by one pump wave has been demonstrated, but not dual channel dropping.

In this letter, we demonstrate, for the first time, one channel and simultaneous two channels dropping based on SFG in a Ti:PPLN waveguide which has two SH phase-matching peaks. The dropped channel (one of two channels or simultaneous two channels) can be selected by tuning of the pump wavelength.

The operation principle of the one channel and simultaneous two channel dropping based on SFG with one pump wave is shown in Fig. 1. If a Ti:PPLN waveguide has two phase-matching curves (SH peaks) such as Fig. 1, one pump wave can interact with two signals at two different phase-matching conditions and make sum frequency (SF); The pump3 interacts with the signal1 in the second phase-matching condition (2_nd phase-matching curve) and the signal2 in the first phase-matching condition (1_st phase-matching curve). However, the signal1 also can interacts with pump1 in the first phase-matching condition. In the same way, the signal2 and the pump2 interact with each other in the second phase-matching condition. Therefore, if we coupled pump1 or pump2 in the Ti:PPLN waveguide, we can make selective one channel (signal1 or signal2) dropping, or the simultaneous two channels dropping can be achieved by the pump3. In the case of single channel dropping (with pump1 or pump2), three different waves interact with one another simultaneously in the Ti:PPLN waveguide and five different waves interact in the process of simultaneous two channel dropping. Some more details will be discussed in the end of this letter.

An 80 mm long Ti:PPLN waveguide of 16.6 µm microdomain period was used to demonstrate all-optical wavelength selective single and dual channel dropping by SFG. One side of the sample was angle polished in order to avoid internal multiple reflection. The waveguide loss was determined to be 0.12 dB/cm at 1.53 µm wavelength (TM-polarization). The operating temperature of the Ti:PPLN waveguide was kept at 158 °C to reduce photorefractive damage [8, 9] and to adjust a SHG phase-matching wavelength of about 1550 nm. The SHG characteristics of the Ti:PPLN waveguide at 158 °C is shown in Fig. 2. The wavelength of the two high peaks are 1550.31 nm and 1550.57 nm, respectively. These two SH peaks correspond to the two different phase-matching curves in Fig. 1. Double or multiple SH peaks are frequently observed in Ti:PPLN waveguides due to unexpected fabrication faults such as non-uniform periodicity of the quasi-phase-matched (QPM) grating or inhomogeneity of the effective refractive index along the waveguide [10]. Those peaks are also observable in intentionally engineered non-uniform QPM gratings [11] and uniform QPM gratings with a temperature gradient [12].

 

Fig. 1. Phase-matching characteristics for channel dropping by SFG. Vertical arrow lines indicate energy conservations.

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The schematic diagram of the experimental setup is shown in Fig. 3. The first signal wave (λ_s1=1556.42 nm) from an extended cavity semiconductor laser (ECL) was combined by a 3 dB-coupler with the second signal wave (λ_s2=1555.85 nm) of a distributed feedback laser (DFB1). The pump wave of DFB2 was amplified by a high power erbium-doped fiber amplifier (HP-EDFA). The polarizations of the all three waves were controlled by fiber optic polarization controllers (PC1, PC2, PC3). They were superimposed by another 3 dB-coupler and launched into the channel waveguide by fiber butt-coupling. The transmitted signal, pump, and generated sum frequency (SF) were observed as a function of the wavelength of pump using an optical spectrum analyzer (OSA). The measured optical spectrums (0.1 nm resolution) are shown in Fig. 4. Without pump, one can clearly see two signals in Fig. 4 (a). When the pump1 is coupled into the Ti:PPLN waveguide with 325 mW, the pump1 (λ_p1=1544.05 nm) and signal1 generate SF signal at about 775.10 nm and simultaneously signal1 is depleted (see Fig. 4 (b)). The extinction ratio of depleted signal was less than -17 dB. The result of signal2 depletion by changing the pump wavelength from λ_p1 to λ_p2(1545.12 nm) is shown in Fig. 4(c). We also observed simultaneous two signal dropping by setting the pump wavelength at 1544.52 nm which is between λ_p1 and λ_p2. In this case, we observed the two SF signal peaks (see the inset of Fig. 4 (d)) which corresponded to the two phase-matching curves in Fig. 1.

Assuming lossless waveguide, we can describe above processes by following the coupled mode equations;

where A ₁, A ₂, A ₃, A ₄, and A ₅ are the field amplitudes ($A_l=\sqrt{n_l⁄{\omega }_l}{E}_l$ where l=1,2,3,4,5) at λ_p, λ_{s 1}, λ_{sf 1}, λ_{s 2}, λ_{sf 2} (p=pump, s1=signal1, sf1=sum frequency1, s2=signal2, sf2=sum frequency2) respectively; ${\kappa }_1\equiv {d\prime }_{\mathit{123}}\sqrt{\left(({\mu }_o⁄{\epsilon }_o){\omega }_{\mathit{1}}{\omega }_{\mathit{2}}{\omega }_{\mathit{3}}\right)⁄\left(n_{\mathit{1}}{n}_{\mathit{2}}{n}_{\mathit{3}}\right)}$, and ${\kappa }_2\equiv {d\prime }_{\mathit{123}}\sqrt{\left(({\mu }_o⁄{\epsilon }_o){\omega }_{\mathit{1}}{\omega }_{\mathit{4}}{\omega }_{\mathit{5}}\right)⁄\left(n_{\mathit{1}}{n}_{\mathit{4}}{n}_{\mathit{5}}\right)}$ are nonlinear coupling constants and Δk ₁ and Δk ₂ are SFG wave vector mismatches at the 1st and the 2nd phase-matching condition, respectively. To calculate wavelength selective single and dual-channel dropping, above all five waves are needed. However, in the case of pump1 (Fig. 4 (b)), signal2 and sum frequency2 terms are negligible in the equations. In the same manner, for the pump2, signal1 and sum frequency1 terms are not considered. Temperature gradient technique can offer a broad or multi-phase matching condition [12] which allows not only dual-channel dropping but also multiple-channel dropping. In order to understand the phenomena in details, the theoretical calculation of these coupled mode equations will be discussed with a temperature gradient technique [13].

 

Fig. 2. The solid line is SHG curve at the operation temperature of 158 °C. The maximum conversion efficiency is measured to be 300 %/W.

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Fig. 3. Schematic diagram of the experimental setup; ECL : extended cavity semiconductor laser, DFB : distributed feedback laser, HP-EDFA : high power erbium-doped fiber amplifer, OSA : optical spectrum analyzer, (PC1, PC2, PC3) : polarization controller.

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Fig. 4. Optical spectrum at the output of the Ti:PPLN waveguide at three different pump wavelength and no pump. The insets show the spectra of generated SF.

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Fig. 5. Signal depletion by SFG versus coupled pump power.

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The signal depletion by SFG as a function of the coupled pump power is shown in Fig. 5. As the coupled pump power increased, it was observed that the signal power monotonically decreased. The coupled pump power of 325 mW was needed for signal depletion to less than -17 dB.

In conclusion, we have demonstrated simultaneous two signal channel dropping using two phase-matching condition in the Ti:PPLN waveguide by SFG for the first time to the best of our knowledge. We selectively dropped one of two single channels and both channels by tuning of the pump wavelength. Less than -17 dB of signal channel dropping extinction ratio have been observed with coupled pump power of 325 mW. However, the extinction ratio of 17 dB is not sufficient for optical communication applications. Further research to improve the extinction ratio and to utilize pulse signals are underway for applications in optical communication network. We believe that this selective channel dropping by SFG might be a useful technology for future all-optical communication network.